Method and apparatus for transmitting downlink in wireless communication system

ABSTRACT

A method for transmitting the downlink in a WLAN that includes the steps of: an access point (AP) transmitting, to a plurality of stations (STA), each of a plurality of request to send (RTS) frames through each of a plurality of channels; and the AP receiving a clear to send (CTS) frame from at least one of the plurality of STAs through at least one channel from the plurality of channels, wherein each of the plurality of RTS frames may include channel information for indicating a channel to be used from among the plurality of channels when transmitting the downlink to each of the STAs, and identifier information for indicating the plurality of STAs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/765,127, filed on Jul. 31, 2015, now U.S. Pat. No. 10,342,045, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2014/000994, filed on Feb. 5, 2014, which claimsthe benefit of U.S. Provisional Application No. 61/761,157, filed onFeb. 5, 2013, the contents of which are all hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and an apparatus for transmitting a downlinkin a wireless local area network (WLAN).

Related Art

A Wireless Next Generation Standing Committee (WNG SC) of institute ofelectrical and electronic engineers (IEEE) 802.11 is an AD-HOC committeethat a next-generation wireless local area network (WLAN) in the mediumand long term.

In an IEEE conference in March 2013, Broadcom presented the need ofdiscussion of the next-generation WLAN after IEEE 802.11ac in the firsthalf of 2013 when an IEEE 802.11ac standard is finished based on a WLANstandardization history. A motion for foundation of a study group whichOrange and Broadcom proposed in the IEEE conference in March 2013 andmost members agreed has been passed.

A scope of a high efficiency WLAN (HEW) which the next-generation WLANstudy group primarily discusses the next-generation study group calledthe HEW includes 1) improving a 802.11 physical (PHY) layer and a mediumaccess control (MAC) layer in bands of 2.4 GHz and 5 GHz, 2) increasingspectrum efficiency and area throughput, 3) improving performance inactual indoor and outdoor environments such as an environment in whichan interference source exists, a dense heterogeneous networkenvironment, and an environment in which a high user load exists, andthe like. That is, the HEW operates at 2.4 GHz and 5 GHz similarly tothe existing WLAN system. A primarily considered scenario is a denseenvironment in which access points (APs) and stations (STAs) are a lotand under such a situation, improvement of the spectrum efficiency andthe area throughput is discussed. In particular, in addition to theindoor environment, in the outdoor environment which is not considerablyconsidered in the existing WLAN, substantial performance improvement isconcerned.

In the HEW, scenarios such as wireless office, smart home, stadium,Hotspot, and building/apartment are largely concerned and discussionabout improvement of system performance in the dense environment inwhich the APs and the STAs are a lot is performed based on thecorresponding scenarios.

In the future, in the HEW, improvement of system performance in anoverlapping basic service set (OBSS) environment and improvement ofoutdoor environment performance, and cellular offloading are anticipatedto be actively discussed rather than improvement of single linkperformance in one basic service set (BSS). Directionality of the HEVmeans that the next-generation WLAN gradually has a technical scopesimilar to mobile communication. When a situation is considered, inwhich the mobile communication and the WLAN technology haven beendiscussed in a small cell and a direct-to-direct (D2D) communicationarea in recent years, technical and business convergence of thenext-generation WLAN and the mobile communication based on the HEW ispredicted to be further active.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting a downlink in awireless communication system.

The present invention also provides an apparatus for transmitting adownlink in a wireless communication system.

In an aspect, a method for transmitting a downlink in a wirelesscommunication system is provided. A method for transmitting a downlinkin a wireless local area network (WLAN) comprises: transmitting, by anaccess point (AP), multiple respective request to send (RTS) frames tomultiple stations (STAs) through multiple respective channels; andreceiving, by the AP, a clear to send (CTS) frame from at least one ofthe multiple STAs through at least one channel of the multiple channels,wherein each of the multiple RTS frames includes channel informationindicating a channel to be used when performing the downlinktransmission to the multiple respective STAs among the multiple channelsand identifier information indicating the multiple STAs.

In another aspect, An access point (AP) transmitting a downlink in awireless local area network (WLAN) comprises a radio frequency (RF) unitconfigured to transmit and receive a radio signal and a processorconfigured to: transmit each of a plurality of request to send (RTS)frames to each of a plurality of stations (STAs) through each of aplurality of channels; and receive a clear to send (CTS) frame from atleast one of the plurality of STAs through at least one channel of theplurality of channels, wherein each of the plurality of RTS framesincludes channel information indicating a channel to be used whenperforming the downlink transmission to the plurality of STAs among theplurality of channels and identifier information indicating theplurality of STAs.

Data transmitting and receiving methods based on FDMA can be usedbetween an extended AP supporting the existing legacy channel band and anewly defined extended channel band and a legacy STA supporting theexisting legacy channel band and an extended STA supporting the existinglegacy channel band and a newly defined extended channel band.Accordingly, a data throughput and frequency efficiency can be increasedby using the newly extended channel band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating a structure of a wirelesslocal area network (WLAN).

FIG. 2 is a diagram illustrating a layer architecture of a WLAN systemsupported by IEEE 802.11.

FIG. 3 is a conceptual diagram illustrating an issue which may occurwhen the STA senses a medium.

FIG. 4 is a conceptual diagram illustrating a method for transmittingand receiving an RTS frame and a CTS frame in order to solve a hiddennode issue and an exposed node issue.

FIG. 5 is a conceptual diagram illustrating information on a bandwidthof the WLAN.

FIG. 6 is a conceptual diagram illustrating a method for transmittingdownlink data by an AP according to the embodiment of the presentinvention.

FIG. 7 is a conceptual diagram illustrating an RTS frame format forsupporting a method for transmitting a downlink based on FDMA accordingto the embodiment of the present invention.

FIG. 8 is a conceptual diagram illustrating the method for transmittinga downlink based on the FDMA according to the embodiment of the presentinvention.

FIG. 9 is a conceptual diagram illustrating the method for transmittinga downlink based on the FDMA according to the embodiment of the presentinvention.

FIG. 10 is a conceptual diagram illustrating the method for transmittinga downlink based on the FDMA according to the embodiment of the presentinvention.

FIG. 11 is a conceptual diagram illustrating a null padding method bythe AP according to the embodiment of the present invention.

FIG. 12 is a conceptual diagram illustrating a method for transmitting adata frame according to the embodiment of the present invention.

FIG. 13 is a conceptual diagram illustrating a frame structure by thetransmission method according to the embodiment of the presentinvention.

FIG. 14 is a conceptual diagram illustrating a PLCP header according tothe embodiment of the present invention.

FIG. 15 is a conceptual diagram illustrating a method for configuring aframe transmission time in the downlink transmitting method based on theFDMA according to the embodiment of the present invention.

FIG. 16 is a block diagram illustrating a wireless apparatus to whichthe embodiment of the present invention can be applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a concept view illustrating the structure of a wireless localarea network (WLAN).

An upper part of FIG. 1(A) shows the structure of the IEEE (institute ofelectrical and electronic engineers) 802.11 infrastructure network.

Referring to the upper part of FIG. 1(A), the WLAN system may includeone or more basic service sets (BSSs, 100 and 105). The BSS 100 or 105is a set of an AP such as AP (access point) 125 and an STA such as STA1(station) 100-1 that may successfully sync with each other tocommunicate with each other and is not the concept to indicate aparticular area. The BSS 105 may include one AP 130 and one or more STAs105-1 and 105-2 connectable to the AP 130.

The infrastructure BSS may include at least one STA, APs 125 and 130providing a distribution service, and a distribution system (DS) 110connecting multiple APs.

The distribution system 110 may implement an extended service set (ESS)140 by connecting a number of BSSs 100 and 105. The ESS 140 may be usedas a term to denote one network configured of one or more APs 125 and130 connected via the distribution system 110. The APs included in oneESS 140 may have the same SSID (service set identification).

The portal 120 may function as a bridge that performs connection of theWLAN network (IEEE 802.11) with other network (for example, 802.X).

In the infrastructure network as shown in the upper part of FIG. 1, anetwork between the APs 125 and 130 and a network between the APs 125and 130 and the STAs 100-1, 105-1, and 105-2 may be implemented.However, without the APs 125 and 130, a network may be establishedbetween the STAs to perform communication. The network that isestablished between the STAs without the APs 125 and 130 to performcommunication is defined as an ad-hoc network or an independent BSS(basic service set).

A lower part of FIG. 1 is a concept view illustrating an independentBSS.

Referring to the lower part of FIG. 1, the independent BSS (IBSS) is aBSS operating in ad-hoc mode. The IBSS does not include an AP, so thatit lacks a centralized management entity. In other words, in the IBSS,the STAs 150-1, 150-2, 150-3, 155-4 and 155-5 are managed in adistributed manner. In the IBSS, all of the STAs 150-1, 150-2, 150-3,155-4 and 155-5 may be mobile STAs, and access to the distributionsystem is not allowed so that the IBSS forms a self-contained network.

The STA is some functional medium that includes a medium access control(MAC) following the IEEE (Institute of Electrical and ElectronicsEngineers) 802.11 standards and that includes a physical layer interfacefor radio media, and the term “STA” may, in its definition, include bothan AP and a non-AP STA (station).

The STA may be referred to by various terms such as mobile terminal,wireless device, wireless transmit/receive unit (WTRU), user equipment(UE), mobile station (MS), mobile subscriber unit, or simply referred toas a user.

FIG. 2 is a view illustrating a layer architecture of a WLAN systemsupported by IEEE 802.11.

FIG. 2 conceptually illustrates a layer architecture (PHY architecture)of a WLAN system.

The WLAN system layer architecture may include an MAC (medium accesscontrol) sub-layer 220, a PLCP (Physical Layer Convergence Procedure)sub-layer 210, and a PMD (Physical Medium Dependent) sub-layer 200. ThePLCP sub-layer 210 is implemented so that the MAC sub-layer 220 isoperated with the minimum dependency upon the PMD sub-layer 200. The PMDsub-layer 200 may serve as a transmission interface to communicate databetween a plurality of STAs.

The MAC sub-layer 220, the PLCP sub-layer 210, and the PMD sub-layer 200may conceptually include management entities.

The management entity of the MAC sub-layer 220 is denoted an MLME (MAClayer management entity, 225), and the management entity of the physicallayer is denoted a PLME (PHY layer management entity, 215). Suchmanagement entities may offer an interface where a layer managementoperation is conducted. The PLME 215 is connected with the MLME 225 tobe able to perform a management operation on the PLCP sub-layer 210 andthe PMD sub-layer 200, and the MLME 225 is also connected with the PLME215 to be able to perform a management operation on the MAC sub-layer220.

There may be an SME (STA management entity, 250) to perform a proper MAClayer operation. The SME 250 may be operated as a layer independentcomponent. The MLME, PLME, and SME may communicate information betweenthe mutual components based on primitive.

The operation of each sub-layer is briefly described below. The PLCPsub-layer 210 delivers an MPDU (MAC protocol data unit) received fromthe MAC sub-layer 220 according to an instruction from the MAC layerbetween the MAC sub-layer 220 and the PMD sub-layer 200 to the PMDsub-layer 200 or delivers a frame from the PMD sub-layer 200 to the MACsub-layer 220. The PMD sub-layer 200 is a PLCP sub-layer and the PMDsub-layer 200 may communicate data between a plurality of STAs by way ofa radio medium. The MPDU (MAC protocol data unit) delivered from the MACsub-layer 220 is denoted a PSDU (Physical Service Data Unit) on the sideof the PLCP sub-layer 210. The MPDU is similar to the PSDU, but in casean A-MPDU (aggregated MPDU), which is obtained by aggregating aplurality of MPDUs, has been delivered, each MPDUs may differ from thePSDU.

The PLCP sub-layer 210 adds an additional field including informationrequired by the physical layer transceiver while receiving the PSDU fromthe MAC sub-layer 220 and delivering the same to the PMD sub-layer 200.In this case, the added field may include a PLCP preamble to the PSDU, aPLCP header, and tail bits necessary to return the convolution encoderto zero state. The PLCP preamble may play a role to allow the receiverto prepare for syncing and antenna diversity before the PSDU istransmitted. The data field may include padding bits to the PSDU, aservice field including a bit sequence to initialize the scrambler, anda coded sequence in which a bit sequence added with tail bits has beenencoded. In this case, as the encoding scheme, one of BCC (BinaryConvolutional Coding) encoding or LDPC (Low Density Parity Check)encoding may be selected depending on the encoding scheme supported bythe STA receiving the PPDU. The PLCP header may include a fieldcontaining information on the PPDU (PLCP Protocol Data Unit) to betransmitted.

The PLCP sub-layer 210 adds the above-described fields to the PSDU togenerate the PPDU (PLCP Protocol Data Unit) and transmits the same to areceiving station via the PMD sub-layer 200, and the receiving stationreceives the PPDU and obtains information necessary for data restorationfrom the PLCP preamble and PLCP header to thus restore the same.

FIG. 3 is a conceptual diagram illustrating an issue which may occurwhen the STA senses a medium.

An upper end of FIG. 3 illustrates a hidden node issue and a FIG. 3(B)illustrates an exposed node issue.

At the upper end of FIG. 3, it is assumed that an STA A 300 and an STA B320 transmit and receive current data and an STA C 330 and an STA B 320has data to be transmitted. When the data is transmitted and receivedbetween the STA A 300 and the STA B 320, a specific channel may be busy.However, when the STA C 330 carrier-senses a medium before transmittingthe data to the STA B 320 due to transmission coverage, the STA C 330may determine that the medium for transmitting the data to the STA B 320is in an idle state. When the STA C 330 determines that the medium is inthe idle state, the data may be transmitted from the STA C 330 to theSTA B 320. Consequently, since the STA B 320 simultaneously receivesinformation of the STA A 300 and the STA C 330, a collision of dataoccurs. In this case, the STA A 300 may be a hidden node as the STA C330.

At a lower end of FIG. 3, it is assumed that an STA B 350 transmits datato an STA A 340. When an STA C 360 intends to transmit data to an STA D370, the STA C 360 may perform carrier sensing in order to find whetherthe channel is busy. The STA C 360 may sense that the medium is busy dueto transmission coverage of the STA B 350 because the STA B 350transmits information to the STA A 340. In this case, although the STA C360 intends to transmit data to the STA D 370, since it is sensed thatthe medium is busy, the STA C 360 may not transmit the data to the STA D370. Until it is sensed that the medium is idle after the STA B 350completes transmitting the data to the STA A 340, a situation in whichthe STA C 360 needs to unnecessarily wait occurs. That is, although theSTA A 340 is out of a carrier sensing range of the STA C 360, the STA A340 may prevent data transmission by the STA C 360. In this case, theSTA C 360 becomes an exposed node of the STA B 350.

In order to solve the hidden nose issue disclosed at the upper end ofFIG. 3 and the exposed node issue disclosed at the lower end of FIG. 3,it may be sensed whether the medium is busy by using an RTS frame and aCTS frame in a WLAN.

FIG. 4 is a conceptual diagram illustrating a method for transmittingand receiving the RTS frame and the CTS frame in order to solve thehidden node issue and the exposed node issue.

Referring to FIG. 4, short signaling frames such as the request to send(RTS) frame and the clear to send (CTS) frame may be used in order tosolve the hidden node issue and the exposed node issue. It may beoverheard whether data is transmitted and received among neighboringSTAs based on the RTS frame and the CTS frame.

An upper end of FIG. 4 illustrates a method for transmitting an RTSframe 403 and a CTS frame 405 in order to solve the hidden node issue.

Assumed that both an STA A 400 and an STA C 420 intend to transmit datato an STA B 410, when the STA A 400 sends the RTS frame 403 to the STA B410, the STA B 410 may transmit the CTS frame 405 to both the STA A 400and the STA C 420 therearound. The STA C 420 that receives the CTS frame405 from the STA B 410 may obtain information indicating that the STA A400 and the STA B 410 are transmitting data. Further, the RTS frame 403and the CTS frame 405 include a duration field including information ona busy duration of a radio channel to configure a network allocationvector (NAV) during a predetermined duration so as to prevent the STA C420 from using the channel.

The STA C 420 waits until the transmission and reception of the databetween the STA A 400 and the STA B 410 is completed, and as a result,the STA C 420 may avoid the collision at the time of transmitting thedata to the STA B 410.

A lower end of FIG. 4 illustrates a method for transmitting an RTS frame433 and a CTS frame 435 in order to solve the exposed node issue.

An STA C 450 overhears transmission of the RTS frame 433 and the CTSframe 435 of an STA A 430 and an STA B 440, and as a result, the STA C450 may find that no collision occurs in spite of transmitting the datato another STA D 460. That is, the STA B 440 transmits the RTS frame 433to all neighboring terminals and transmits the CTS frame 435 to only theSTA A 430 to which the STA B 440 needs to actually transmit data. Sincethe STA C 450 receives only the RTS frame 433 and may not receive theCTS frame 435 of the STA A 430, it may be found that the STA A 430 isout of a carrier sensing range of the STA C 450. Accordingly, the STA C450 may not transmit data to the STA D 460.

An RTS frame format and a CTS frame format are disclosed in 8.3.1.2 RTSframe format and 8.3.1.3 CTS frame format of “IEEE Standard forInformation Technology Telecommunications and information exchangebetween systems Local and metropolitan area networks Specificrequirements Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications which is IEEE DraftP802.11-REVmb™/D12 opened in November 2011.

The IEEE 802.11 WLAN (wireless local area network, WLAN) standard mayhave different transmission rate in various bands. A very highthroughput (VHT) system which is the IEEE 802.11ac standard is toprovide a throughput of 1 Gbps or more at a MAC service access point(SAP).

To this end, the VHT system may support a channel bandwidth of 80/160MHz and 8 spatial streams. When the channel bandwidth of 160 MHz, 8spatial streams, 256QAM, and a short guard interval (GI) are allimplemented, the VHT system may provide a maximum of 6.9 Gbpstransmission rate.

However, VHT STAs that support multiple VHT systems need tosimultaneously use the channel in order for an aggregated throughput ofa VHT BSS to satisfy 1 Gbps in an actual environment.

An AP that supports a VHT may use space division multiple access (SDMA)or multi user-multiple input multiple output (MU-MIMO) as a method fortransmitting data in order for the STAs supporting the multiple VHTsystems to simultaneously use the channel That is, the VHT systems maysimultaneously transmit and receive different data among the multipleVHT STAs and the VHT AP based on multiple spatial streams.

In the VHT system, since legacy stations (STAs) supporting IEEE802.11a/n are widely used in transmitting data by using the channelbandwidth of 160 MHz.

Accordingly, non-contiguous channels need to be aggregated and used.

FIG. 5 is a conceptual diagram illustrating information on the bandwidthof the WLAN.

With the increase in demand for high-definition multimedia transmission,a channel bandwidth for the WLAN has been extended.

Referring to FIG. 5, channel bands which the IEEE 802.11ac may currentlyuse and bands to be newly added in a frequency band of 5 GHz areillustrated.

When channel bands to be newly allocated for the WLAN in channel bandsof 5350 MHz to 5470 MHz and 5825 MHz to 5925 MHz are considered, thenumber of channel bands which may be used by the STA or AP may increase.For example, referring to the 80 MHz channel band, 80 MHz channel bandswhich are usable may increase from 6 channels at present to 9 channelsby allocating the new channel bands. As another example, referring tothe 160 MHz channel band, 160 MHz channel bands which are usable mayincrease from 2 channels at present to 4 channels.

The legacy STA (e.g., the STA supporting the IEEE 802.11a/n/ac) in therelated art, which operates at 5 GHz does not support the newlyallocated channels. Accordingly, the AP that supports the newlyallocated channels configures a primary channel as a channel in whichthe legacy STA may operate to support the legacy STA.

Hereinafter, in the embodiment of the present invention, the channelbands allocated to 5 GHz are defined and disclosed as two types ofchannel bands. The existing channel supported by the legacy STA such asthe IEEE 802.11a/n/ac is defined as a term called a legacy channel bandthe newly allocated channel band at 5 GHz is defined as an extendedchannel band. Further, an STA that may operate in the extended channelband is used as a term such as an extended STA.

Hereinafter, in the embodiment of the present invention, a method willbe disclosed, in which the AP performs downlink channel transmission tothe legacy STA and the extended STA by using the legacy channel band inthe 5 GHz frequency band and the extended channel band. The AP mayperform downlink transmission based on frequency division multipleaccess (FDMA). A legacy channel and an extended channel may be thenon-contiguous channel or a contiguous channel.

The embodiment of the present invention may be applied to the FDMA baseddownlink transmission when all channels are the legacy channels and theFDMA based downlink transmission when all channels are the extendedchannels in addition to the FDMA based downlink transmission through thelegacy channel and the extended channel.

FIG. 6 is a conceptual diagram illustrating a method for transmittingdownlink data by the AP according to the embodiment of the presentinvention.

In FIG. 6, a method is disclosed, in which the AP transmits data to thelegacy STA and the extended STA in respective 80 MHz channel bands byusing an 80 MHz legacy channel band (hereinafter, referred to as a 80MHz legacy channel) and an 80 MHz extended channel band (hereinafter,referred to as a 80 MHz extended channel). The 80 MHz legacy channel mayinclude 4 20 MHz legacy channel bands (hereinafter, referred to as a 20MHz legacy channel). The 80 MHz extended channel may include 4 20 MHzextended channel bands (hereinafter, referred to as a 20 MHz extendedchannel). Hereinafter, in the embodiment of the present invention, therespective channels are disclosed as the divided channel bands for easydescription, but may be regarded as multiple channel bands withoutdividing the channel bands. The multiple channels may include 8 20 MHzchannels.

The AP may configure one 20 MHz legacy channel among 4 20 MHz legacychannels included in the 80 MHz legacy channels as the primary channel.The AP may perform a back-off procedure for channel access with respectto the configured primary channel.

The AP may verify a channel state of a secondary channel during a PIFSinterval before a back-off timer expires. The secondary channel mayinclude residual 20 MHz legacy channels and 20 MHz extended channelsincluded in the 80 MHz legacy channel and the 80 MHz extended channelother than the primary channel.

The AP may determine the state of the channel for the point coordinationfunction (PCF) interframe space) time before starting TXOP in order todetermine whether the second channel is idle or busy.

When the secondary channel is idle for the PIFS time, the AP maydetermine that the state of the secondary channel is idle. In FIG. 6, itis described that it is assumed that all secondary channels are idle foreasy description, but a channel determined as a busy state among thesecondary channel may be present.

The AP may transmit an RTS frame 600 in a duplicated PHY protocol dataunit (PPDU) format through the primary channel which the AP accesses andthe respective secondary channels of which channel states are determinedas the idle state. The RTS frame 600 in the duplicated PHY protocol dataunit (PPDU) format may be a form acquired by duplicating an RTS frameformat transmitted in the 20 MHz channel. The AP may transmit the RTSframe 600 at a similarly set time.

In FIG. 6, since all of 7 secondary channels are in the idle state, theAP may transmit the RTS frame 600 in the duplicated PHY protocol dataunit (PPDU) format in 8 20 MHz channels including the primary channeland the secondary channels.

The RTS frame 600 transmitted by the AP may include information on anSTA that receives the RTS frame 600 and information on a receptionchannel which the STA will use. The STA that receives the RTS frame maybe the legacy STA and/or the extended STA. For example, the RTS frame600 may include a list of STAs that receive the RTS frame 600 and a listof channels which the AP uses to transmit the downlink data to the STAaccording to the embodiment.

FIG. 7 is a conceptual diagram illustrating the RTS frame format forsupporting a method for transmitting a downlink based on the FDMAaccording to the embodiment of the present invention.

Referring to FIG. 7, the RTS frame (hereinafter, referred to as ‘RTSframe) for supporting the downlink transmitting method based on the FDMAmay include a frame control field 700, a duration field 710, a receiveraddress field 720, a transmitter address field 730, and a frame checksequence (FCS) field 740.

The frame control field 700 may include information for indicating atransmitted frame type. For example, the frame control field 700 mayinclude information for indicating that the transmitted RTS frame is aframe used to start the downlink transmission based on the FDMA.

The duration field 710 may include information for updating the networkallocation vector (NAV) of the STA that receives the RTS frame.Alternatively, the duration field 710 may be used to configureinformation (e.g., a transmission start time and/or a transmission endtime) associated with a transmission time of the transmitted andreceived frame in the embodiment of the present invention. For example,the duration field 710 may be used to configure a time when the STAtransmits the CTS frame. Alternatively, the duration field 710 mayinclude information on a maximum transmission time (alternatively, amaximum transmission duration) of a frame transmitted after the same RTSframe as the CTS frame. The STA that receives the RTS frame may transmitthe CTS frame based on the duration field.

The transmitter address field 730 may include information on an addressof the STA that transmits the RTS frame.

The FCS field 740 may include information for discovering an error whichoccurs in the frame.

The receiver address field 720 may include information on the STA thatreceives the RTS frame and information on a reception channel which theSTA will use at the time of receiving the downlink data. For example,the receiver address field may include an STA identifier informationfield 750 and a channel information field 760. When multiple (e.g., 4)STAs that will receive the downlink data are present, multiple STAidentifier information fields may be allocated. The channel informationfield may include information on a channel allocated when the multipleSTAs that receives the downlink data receives the downlink data.

For example, a first STA identifier information 750-1 may includeinformation on a partial AID other than 1 most significant bit (MSB)among 11 association identifier (AID) bits used as identifierinformation of a first STA. Similarly, a second STA identifierinformation 750-2 may include information on a partial AID of a secondSTA, a third STA identifier information 750-3 may include information ona partial AID of a third STA, and a fourth STA identifier information750-4 may include information on a partial AID of a fourth STA.

According to the embodiment of the present invention, the STA thatreceives the RTS frame transmitted by the AP may determine that thereceived RTS frame is the RTS frame transmitted from the AP thatperforms the downlink transmission based on the FDMA by consideringinformation on a frame type of the RTS frame. Further, the STA mayidentify that the identifier information included in the receiveraddress field 720 is partial AID information based on the typeinformation of the RTS frame. It may be determined whether the STA isthe STA that receives the downlink data from the AP based on the partialAID information included in the RTS frame.

As another embodiment, a value of the AID of the STA that supports thedownlink transmitting method based on the FDMA may be configured as avalue that belong to a specific range such as 1 t 1024 or 1025 to 2048.It may be assumed that the value of the AID of the STA has the value inthe range of 1 to 1024 and the RTS frame received by the STA is the RTSframe for the downlink transmission based on the FDMA. In this case, theSTA may find that the receiver address field of the RTS frame includesinformation (e.g., a list of partial AIDs) on the partial AID. The STAmay determine all AID values from the partial AIDs by configuring theMSB of the partial AID included in the receiver address field as 0. TheSTA may determine whether the STA is designated as an STA that willreceive data from the AP based on all determined AIDs.

A first channel information field 770-1 included in the receiver addressfield 720 may include information on a channel bandwidth allocated tothe first STA and a second channel information field 770-2 may includeinformation on a channel bandwidth allocated to the second STA. When itis assumed that the second STA uses a channel adjacent to the first STA,offset information between a first channel allocated to the first STAand a second channel allocated to the second STA may not be separatelytransmitted in the channel information field. For example, the firstchannel information field 770-1 may include channel band informationbased on the primary channel and the second channel information field770-2 may be a channel field just adjacent to the first channelallocated based on the first channel information field. When the firstchannel and the second channel are not adjacent channels, information ona channel offset is additionally transmitted to the channel informationfield to transmit information on the position of the second channelbased on the first channel.

Similarly, a third channel information field 770-3 may includeinformation on a channel bandwidth allocated to the third STA and afourth channel information field 770-4 may include information on achannel bandwidth allocated to the fourth STA.

FIG. 8 is a conceptual diagram illustrating a method for transmitting adownlink based on the FDMA according to the embodiment of the presentinvention.

In FIG. 8 it is exemplarily described that it is assumed that downlinktransmission channels are allocated to 3 STAs.

Referring to FIG. 8, the first STA is allocated with the 80 MHz legacychannel to receive data from the AP. The second STA is allocated withthe 40 MHz extended channel and the third STA is allocated with thesecond 20 MHz extended channel to receive data from the AP.

The receiver address field of the RTS frame transmitted by the AP mayinclude the STA identifier information field and the channel informationfield.

First STA identifier information 810 may include information on apartial AID (e.g., 10 bits) of the first STA, second STA identifierinformation 820 may include information on a partial AID of the secondSTA, and third STA identifier information 830 may include information ona partial AID of the third STA. When fourth STA identifier informationis configured in the receiver address field as illustrated in FIG. 7,the fourth STA is not present, and as a result, the fourth STAidentifier information 840 may be configured as a reserved value.

Further, a first channel information field 850 of the receiver addressfield may include information on a downlink channel bandwidth allocatedto transmit data to the first STA, a second channel information field860 may include information on a downlink channel bandwidth allocated totransmit data to the second STA, and a third channel information field870 may include information on a downlink channel bandwidth allocated totransmit data to the third STA. For example, when it is assumed that thechannel information field is ‘10’ as information of 2 bits, the channelinformation field indicates the 80 MHz channel bandwidth, when thechannel information field is ‘01’, the channel information fieldindicates the 40 MHz channel bandwidth, and when the channel informationfield is ‘00’, the channel information field indicates the 20 MHzchannel bandwidth, ‘10’ may be configured in the first channelinformation field, ‘01’ may be configured in the second channelinformation field, and ‘00’ may be configured in the third channelinformation field. Since the fourth STA is not present, a fourth channelinformation field 880 may be configured as ‘11’ which is a reservedvalue.

The receiver address field disclosed in FIGS. 7 and 8 is one exemplaryinformation format for transmitting information on the STA that receivesthe RTS frame and information on the downlink channel which the AP usesto transmit the downlink data to the STA. That is, the receiver addressfield may be implemented in various formats in order to transmit theinformation on the STA that receives the RTS frame and the informationon the downlink channel used to transmit the data to the STA. Forexample, when a list of the multiple STAs is configured and managed bythe unit of a group, a value of an identifier for a specific STA groupmay be configured as the receiver address field of the RTS frame. Forexample, when the first STA, the second STA, the third STA, and thefourth STA are configured as one group, identifier information (e.g.,group ID 10) of one configured group may be used while being included inthe receiver address field of the RTS frame.

Referring back to FIG. 6, the STA that receives the RTS frame 600 mayidentify that the STA corresponds to an STA that receives the downlinktransmission data from the AP based on the STA identifier informationincluded in the RTS frame. Further, when the STA that receives the RTSframe 600 is the STA that receives the downlink transmission data, theSTA may move to a reception channel indicated based on the channelinformation field included in the RTS frame 600. When a current channelof the STA is the reception channel indicated based on the channelinformation field, the STA may receive data transmitted from the AP inthe current channel.

The STA may transmit the CTS frame 610 to the AP in the receptionchannel indicated based on the channel information field as a responseto the received RTS frame 600. The CTS frame 610 may be the CTS frame610 in the duplicated PPDU format. Different STAs may transmit the CTSframe 610 based on transmission time information of the CTS frameincluded in the RTS frame transmitted by the AP. Each of the multipleSTAs may transmit the CTS frame 610 based on the maximum transmissiontime (alternatively, maximum transmission duration) of the CTS frame 610included in the duration field of the received RTS frame 600. Durationfields of the RTS frames 600 which the AP transmits to the multiple STAsmay be configured as the same value and the multiple STAs may transmitthe CTS frame 610 based on the same duration field value.

For example, the STA may configure a time to transmit the CTS frame 610based on the duration field information included in the RTS frame 600.

The channel bandwidth of the channel in which the CTS frame 610 istransmitted may be determined based on the channel information fieldincluded in the RTS frame 600. That is, the channel bandwidth in whichthe STA transmits the CTS frame 610 may be a channel bandwidth allocatedto the STA through the RTS frame 600. The channel bandwidth allocatedfor the STA to transmit the CTS frame 610 may be not larger than thechannel bandwidth allocated to the STA through the RTS frame 600.

The AP that receives the CTS frame 610 from the first STA and the secondSTA may downlink-transmit data frames 630 and 650 to the first STA andthe second STA. The AP may transmit the data frame 650 to the first STAthrough 4 respective 20 MHz legacy channels included in the 80 MHzlegacy channel. The AP may transmit the data frame 630 to the second STAthrough 4 respective 20 MHz extended channels included in the 80 MHzextended channel.

When the AP transmits the data frames 630 and 650 to the STA, the sizesof the data frames transmitted to the respective STAs may be differentfrom each other and modulation coding schemes (MCS) used to transmit thedownlink data may be different from each other. Accordingly, an issue inwhich transmission durations required for the AP to transmit the dataframes 630 and 650 to the respective STAs are different from each othermay occur. In the embodiment of the present invention, it may beimplemented in such a manner that the transmission durations when the APtransmits the data frame to the respective STAs are configured to be thesame as each other.

The AP may configure a transmission end time of the data frame similarlyat the time of transmitting the data frames 630, 640, and 650 to thefirst STA and the second STA. For example, when the AP first completestransmission of the effective data frame 630 to the second STA, the APmay transmit the data frame 640 which is null padded to the second STAuntil transmission of the data frame 650 to the first STA ends. By usingsuch a method, a time at which the AP completes the transmission of thedata frame 650 to the first STA and a time at which the AP completes thetransmission of the data frames 630 and 640 to the second STA may beconfigured to be the same as each other.

The first STA and the second STA may transmit block ACKs 650 and 660 asresponses to the data frames 630 and 650 transmitted from the AP. Theblock ACKs 650 and 660 may be transmitted in the respective channelsallocated to the first STA and the second STA. For example, the firstSTA may transmit the block ACK 650 through 4 respective 20 MHz legacychannels included in the 80 MHz legacy channel. Further, the second STAmay transmit the block ACK 660 through 4 respective 20 MHz extendedchannels included in the 80 MHz extended channel.

The first STA and the second STA may determine the transmission time ofthe block ACKs 650 and 660 based on the fields (e.g., duration fields)included in the data frames 630 and 650 which the AP transmits to thefirst STA and the second STA.

In detail, the respective STAs that receive the data frame in therespective channels transmit the block ACK to the AP through theallocated channels to transmit information regarding whether to receivethe data frame to the AP. In this case, since the respective STAstransmit the block ACK to the AP by using different channels, times whenthe respective STAs transmit the block ACK need to be the same as eachother. To this end, the AP may transmit the data frame including theinformation regarding the transmission time of the block ACK to therespective STAs in order to adjust the transmission time of the blockACK to be the same as each other.

As illustrated in FIG. 6, it may be assumed that the AP transmits thefirst data frame 650 to the first STA through the legacy channel andtransmits the second data frame to the second STA through the extendedchannel. In this case, information included in the duration field of thefirst data frame transmitted to the first STA and the second STA andinformation included in the duration fields of the second data frames630 and 640 may be configured as the same value. The duration field mayinclude information on the transmission time of the block ACK which theSTA will transmit after receiving the data frame. Each of the first STAand the second STA may transmit the block ACK based on the informationon the duration field of the received data frame.

In the embodiment of the present invention, it is disclosed that thetransmission times of the block ACK by different STAs are configured tobe the same as each other based on the field included in the data frame,but the transmission times of the block ACK may be configured to be thesame as each other by using various other methods (e.g., fieldinformation included in another frame).

In the embodiment of the present invention, the respective channels aredisclosed as the divided channel bands for easy description, but may beregarded as multiple channel bands without dividing the channel bands.

For example, the AP may transmit multiple RTS frames to multiple STAsthrough multiple channels and the AP may receive the CTS frame from atleast one of the multiple STAs through at least one channel of themultiple channels.

The CTS frame may be transmitted from at least two STAs of the multipleSTAs to the AP through at least two channels of the multiple channels,respectively. In this case, the AP may transmit multiple data frames toat least two STAs of the multiple STAs. The multiple respective dataframes may be transmitted to correspond to at least two STAs of themultiple STAs, respectively.

FIG. 9 is a conceptual diagram illustrating a method for transmitting adownlink based on the FDMA according to the embodiment of the presentinvention.

Unlike FIG. 6, in FIG. 9, the first STA that operates in the primarychannel may transmit the CTS frame by using both the legacy channel andthe extended channel.

The AP may transmit the RTS frame to the first STA and the second STA.Alternatively, the AP may transmit the RTS frame to only the first STA.The first STA and the second STA may support both the legacy channel andthe extended channel. In FIG. 9, it is assumed that the AP transmits theRTS frame to the first STA and the second STA.

The AP may transmit the RTS frame in the duplicated frame format throughthe entire channel bandwidth (the legacy channel and the extendedchannel to perform the downlink transmission. A terminal that receivesthe RTS frame may verify an available channel during the PIFS beforetransmitting the CTS frame and transmit the CTS frame through theavailable channel.

According to the embodiment of the present invention, the first STA thatoperates in the primary channel may determine information regardingwhether the entire bandwidth to perform the downlink transmission isavailable. When the entire bandwidth is available, the first STA maytransmit the CTS frame with respect to the entire bandwidth for thesecond STA. The second STA may not transmit a separate CTS frame.

The AP may receive the CTS frame from the first STA and transmit thedata frames through the channels allocated to the first STA and thesecond STA, respectively. The transmission times of the data frames mayhave the same value. A data frame in which transmission of effectivedata is first completed may include null padding. The respective STAsthat receive the data frames may transmit the block ACKs to the AP. TheAP may transmit the respective data frames including information forconfiguring the times when the respective STAs transmit the block ACKsto be the same as each other.

Hereinafter, in the embodiment of the present invention, a method forperforming the downlink transmission based on the FDMA by using theexisting RTS frame and CTS frame formats will be disclosed.

FIG. 10 is a conceptual diagram illustrating the method for transmittinga downlink based on the FDMA according to the embodiment of the presentinvention.

In FIG. 10, disclosed is a method for transmitting a downlink by the APwhen the AP finds an operating channel of a specific STA. The AP mayobtain information on the operating channel of the specific STA based onvarious methods. For example, the AP may obtain the information on theoperating channel of the specific STA based on information in which theAP previously performs network with the specific STA or obtaininformation on a current operating channel of the specific STA based onthe current operating channel of the STA, which the specific STAtransmits or a movement operating channel of the STA.

Hereinafter, in the embodiment of the present invention, a downlinktransmitting operation of the AP when the extended STA in which theextended channel is available notifies to the AP that the operatingchannel moves to the extended channel will be disclosed.

Even in FIG. 10, a method is described, in which the AP transmits datato the legacy STA and the extended STA in respective 80 MHz channelbands by using the 80 MHz legacy channel (hereinafter, referred to asthe 80 MHz legacy channel) and the 80 MHz extended channel (hereinafter,referred to as the 80 MHz extended channel). The 80 MHz legacy channelmay include 4 20 MHz legacy channels. Further, the 80 MHz extendedchannel may include 4 20 MHz extended channels.

The AP may perform the back-off procedure for channel access withrespect to the configured primary channel. The AP may verify the channelstate of the secondary channel during a predetermined time interval(e.g., PIFS) before starting the TXOP as the back-off timer expires. TheAP may determine the state of the channel for the PIFS time in order todetermine whether the secondary channel is idle or busy.

When the secondary channel is idle for the PIFS time, the AP maydetermine that the state of the secondary channel is idle. Even in FIG.10, it is described that it is assumed that all secondary channels areidle for easy description.

The AP may transmit RTS frames 1000 and 1010 through the primary channelwhich the AP accesses and the respective primary channel and thesecondary channels of which channel states are determined as the idlestate.

A receiver address field of a first RTS frame 1010 which the APtransmits through the legacy channel may include identifier information(e.g., an MAC address of the first STA) of the first STA that mayoperate in the legacy channel.

A receiver address field of a second RTS frame 1000 which the APtransmits through the extended channel may include identifierinformation (e.g., an MAC address of the second STA) of the second STAthat may operate in the extended channel. The second STA may transmitinformation indicating that the second STA moves to the extended channelto the AP in advance. The AP may transmit the second RTS frame 1000 tothe second STA through the extended channel based on the information.

The first STA that receives the first RTS frame 1010 from the AP and Thesecond SA that receives the second RTS frame 1000 from the AP maytransmit CTS frames 1020 and 1030 to the respective channels throughwhich the first and second STAs receive the RTS frames 1000 and 1010.The first STA and the second STA may transmit the CTS frames 1020 and1030 at configured times. For example, the AP may configure transmissiontimes of the CTS frames 1020 and 1030 by the first STA and the secondSTA based on information (e.g., information on the duration field)included in the RTS frames 1020 and 1030. For example, the durationfields of the first RTS frame 1010 which the AP transmits to the firstSTA and the second RTS frame 1020 which the AP transmits to the secondSTA may be configured as the same value. The information on the durationfield included in each RTS frame may information on a maximumtransmission time (alternatively, a maximum transmission duration) whenthe CTS frames 1020 and 1030 transmitted by each STA are transmitted.

The first STA and the second STA may transmit the CTS frames 1020 and1030 in the duplicated PPDU format to the AP according to the channelbandwidth.

The AP that receives the CTS frames 1020 and 1030 from the first STA andthe second STA may perform the downlink transmission through thechannels allocated to the first STA and the second STA. In detail, theAP may transmit the data frame 1050 to the first STA through the legacychannel and the AP may transmit the data frames 1040 and 1060 to thesecond STA through the extended channel. The AP may adjust transmissiontimes when the AP transmits the data frames 1040, 1050, and 1060 to therespective STAs to be the same as each other. For example, the AP mayadjust the transmission times of the data frames 1040, 1050, and 1060 tothe multiple STAs by using the method such as the null padding.

Similarly as described above, the AP may transmit the data frameincluding the information regarding the transmission time of the blockACK to the respective STAs in order to adjust the transmission time ofthe block ACK to be the same as each other. Each STA may transmit theblock ACK based on the information associated with the transmission timeof the block ACK included in the data frame.

FIG. 11 is a conceptual diagram illustrating a null padding method bythe AP according to the embodiment of the present invention.

Referring to FIG. 11, null padding may be implemented on an MAC layer inan aggregated MAC protocol data unit (A-MPDU) format.

The AP may transmit the data frame in the A-MPDU format acquired byaggregating the MPDU. The null padding may be implemented bytransmitting only a subframe header of the A-MPDU.

Each A-MPDU format may include multiple A-MPDU subframes. Each A-MPDUsubframe may include an MPDU delimiter field 1100, the MPDU, and apadding bit.

The MPDU delimiter field 1100 may include an MPDU length field 1110, acyclic redundancy check (CRC) 1120, and a delimiter signature field1130.

The length field 1110 may include information on the length of the MPDU,the CRC 1120 may include information for error checking, and thedelimiter signature field 1130 may include information for scanning anMPDU delimiter.

According to the embodiment of the present invention, the AP may encodeand fill multiple A-MPDU subframes positioned at a temporally lowerpriority in the A-MPDU frame format with only the MPDU delimiter fieldfor the null padding. For example, the AP repeatedly transmits the MPDUdelimiter in which the MPDU length field is configured as 0 to performthe null padding.

That is, a time when transmission of the multiple data framestransmitted by the AP ends may be the same as a time when transmissionof a maximum interval transmission data frame ends. The maximum intervaltransmission data frame may be a frame in which effective downlink datais transmitted by the AP during the longest interval. The effectivedownlink data may be data which needs to be actuallydownlink-transmitted to the STA. The effective downlink data may bedownlink data which is not null-padded. For example, the effectivedownlink data may be transmitted while being included in the MPDU.Residual data frames other than the maximum interval transmission dataframe among the multiple data frames may be null-padded.

FIG. 12 is a conceptual diagram illustrating a method for transmitting adata frame according to the embodiment of the present invention.

In the downlink transmitting method based on the FDMA according to theembodiment of the present invention, guard intervals used in the dataframes transmitted to the respective STAs may be configured as the sameguard interval. The reason is that since a long guard interval (LGI)adopts a guard interval of 0.8 us and a short guard interval (SGI)adopts a guard interval of 0.4 us, when a specific channel uses LGI andanother channel uses the SGI, the times when the AP completes thedownlink transmission may be the same as each other.

According to the embodiment of the present invention, all downlinkchannels use the same guard interval to configure the completion timesof transmission of the data frames to the multiple STAs to be the sameas each other. The guard intervals used in the respective data framestransmitted to the multiple STAs may be selected as one of the SGI andthe LGI. That is, the guard interval of the data frame which the APtransmits to each STA at a specific time may be selected and used as oneof the SGI and the LGI.

In detail, referring to an upper end of FIG. 12, when a first data frame1210 transmitted to the first STA is configured to use the SGI, a seconddata frame 1220 transmitted to the second STA may be configured to usethe SGI.

On the contrary, referring to a lower end of FIG. 12, when a first dataframe 1250 transmitted to the first STA is configured to use the LGI, asecond data frame 1260 transmitted to the second STA may be configuredto use the LGI.

It may be assumed that the AP transmits data to the STA by using atransmission method such as multi-user (MU)-multiple input multipleoutput (MIMO). In this case, the AP may transmit multiple spatialstreams to the first STA and the second STA. As described above, when itis assumed that the AP transmits data to the first STA based on 4 20 MHzlegacy channels and the AP transmits data to the second STA based on 420 MHz extended channels, the AP may transmit the data to the first STAthrough 4 spatial streams and to the second STA through 4 anotherspatial streams.

When a transmission method using MIMO, the number of long trainingfields (LTF) which are fields used for channel prediction, andsynchronization of a frequency and the time, which are included in thedata frame may vary depending on the number of spatial streams.

FIG. 13 is a conceptual diagram illustrating a frame structure by thetransmission method according to the embodiment of the presentinvention.

Referring to a lower end of FIG. 13, the data frame which the APtransmits to the first STA based on the legacy channel is illustrated.It may be assumed that the AP uses two spatial streams at the time oftransmitting data to the first STA based on the legacy channel In thiscase, two LTFs 1300 are included in the data frame transmitted by the APto be generated.

It is assumed that the AP uses one spatial stream at the time oftransmitting the data frame to the second STA based on the extendedchannel. When one spatial stream is used, one LTF may be included in thedata frame. In this case, transmission completion times of the dataframes which the AP transmits to the first STA and the second STA may bedifferent from each other. For example, the LTF uses the LGI duringtransmission and when the SGI is used during transmitting the data, thecompletion time of the transmission of the data frame to the first STAand the completion time of the transmission of the data frame to thesecond STA may be different from each other. Therefore, in theembodiment of the present invention, when the numbers of spatial streamsused to transmit the data frame to the multiple STAs are different fromeach other, a dummy LTF 1350 is added to the data frame which may betransmitted. For example, the dummy LTF 1350 may be included in a PLCPpreamble or a PLCP header of the data frame. By using such a method, thecompletion times of the downlink transmission to the multiple STAs maybe configured to be the same as each other.

That is, in addition to an LTF including a channel prediction sequencerequired for a spatial stream actually transmitted to the STA, the LTFis additionally inserted in order to adjust the numbers of LTFs to bethe same as each other in respective channels.

As illustrated in FIG. 12, the number of spatial streams actuallytransmitted to the second STA is one, but one dummy LTF 1350 may beadded to the data frame. That is, the AP may generate the data frames sothat the numbers of LTFs included in the respective data framestransmitted to the first STA and the second STA are the same as eachother as two. The AP may configure the number of LTFs included in thedata frame based on the maximum number of spatial streams used fortransmission to a specific STA.

FIG. 14 is a conceptual diagram illustrating the data frame according tothe embodiment of the present invention.

Referring to FIG. 14, the data frame may include information associatedwith a dummy LTF in an SIG field 1400.

For example, the SIG field 1400 may include the number of spatialstreams used to actually transmit the data frame and the total number ofLTFs in order to support the dummy LTF. When the case illustrated inFIG. 13 is assumed, the SIG field 1400 of the data frame transmitted tothe second STA may include information indicating that the number ofspatial streams is one and the total number of LTFs is two.

As another embodiment, the SIG field 1400 may directly include thenumber of dummy LTFs instead of the total number of LTFs. For example,when the case illustrated in FIG. 13 is assumed, the SIG field 1400 ofthe data frame transmitted to the second STA may include informationindicating that the number of dummy LTFs is one.

The SIG field is one example and the SIG field 1400 according to theembodiment of the present invention may include information on thenumber of used spatial streams, information (e.g., information regardingpresence of the dummy LTF and information to determine the number ofdummy LTFs) associated with the dummy LTF.

FIG. 15 is a conceptual diagram illustrating a method for configuring aframe transmission time in the downlink transmitting method based on theFDMA according to the embodiment of the present invention.

In FIG. 15, disclosed is a method in which the multiple STAs transmitthe block ACK to the AP at a configured time, but such a method may alsobe used for the multiple STAs and multiple APs to start or completetransmission of different data or signals at the configured time.

Referring to FIG. 15, the respective STAs that receive the data frame inthe respective channels transmit the block ACKs to the AP through theallocated channels to notify whether to receive the data frame. When theSTAs transmit the block ACKs by using different channels, thetransmission times of the block ACKs need to be configured to be thesame as each other.

Information on the transmission times of the block ACKs transmitted bythe multiple STAs is included in the data frame which may be transmittedin order to configure the transmission times of the block ACKstransmitted by the multiple STAs to be the same as each other.

In detail, the respective STAs that receive the data frame in therespective channels transmit the block ACKs to the AP through theallocated channels to transmit information regarding whether to receivethe data frame to the AP. In this case, since the respective STAstransmit the block ACKs to the AP by using different channels, the timeswhen the respective STAs transmit the block ACK need to be the same aseach other. To this end, the AP may transmit the data frame includingthe information regarding the transmission time of the block ACK to therespective STAs in order to adjust the transmission time of the blockACK to be the same as each other.

It may be assumed that the AP transmits the data frame to the first STAthrough the legacy channel and the AP transmits the data frame to thesecond STA through the extended channel. In this case, a field (e.g.,duration field) 1500 of the first data frame transmitted to the firstSTA may include information on a time 1550 when the first STA transmitsthe block ACK as a response to the data frame. Further, a field (e.g.,duration field) 1500 of the second data frame transmitted to the secondSTA may include information on a time 1550 when the second STA transmitsthe block ACK as a response to the data frame. For example, the durationfields included in the first data frame and the second data frame mayinclude the same value. In this case, the first STA and the second STAmay obtain information on the transmission time of the block ACK basedon the duration fields included in the received data frames and transmitthe block ACK.

FIG. 16 is a block diagram illustrating a wireless device to which anembodiment of the present invention may apply.

Referring to FIG. 16, the wireless device may be an STA that mayimplement the above-described embodiments, and the wireless device maybe an AP 1650 or a non-AP STA (or STA)(1600).

The STA 1600 includes a processor 1610, a memory 1620, and an RF (RadioFrequency) unit 1630.

The RF unit 1630 may be connected with the processor 1620 totransmit/receive radio signals.

The processor 1620 implements functions, processes, and/or methods asproposed herein. For example, the processor 1620 may be implemented toperform the operation of the above-described wireless device accordingto an embodiment disclosed in FIG. 6 to FIG. 15 of the presentinvention.

For example, the processor 1620 may determine the time to transmit theCTS frame based on the RTS frame transmitted by the AP. Further, theprocessor 1620 may determine a channel to receive downlink data based onthe received RTS frame.

The AP 1650 includes a processor 1660, a memory 1670, and an RF (RadioFrequency) unit 1680.

The RF unit 1680 may be connected with the processor 1660 totransmit/receive radio signals.

The processor 1660 implements functions, processes, and/or methods asproposed herein. For example, the processor 1660 may be implemented toperform the operation of the above-described wireless device accordingto an embodiment disclosed in FIG. 6 to FIG. 15 of the presentinvention.

For example, the processor 1660 may transmit multiple RTS frames tomultiple STAs through multiple channels, respectively. Further, theprocessor 1660 may be implemented to receive the CTS frame from at leastone of the multiple STAs through at least one channel of the multiplechannels. Each of the multiple RTS frames may include channelinformation indicating a channel to be used when performing downlinktransmission to the multiple STAs among the multiple channels andidentifier information indicating the multiple STAs.

The processor 1610, 1620 may include an ASIC (Application-SpecificIntegrated Circuit), other chipset, a logic circuit, a data processingdevice, and/or a converter that performs conversion between a basebandsignal and a radio signal. The memory 1620, 1670 may include a ROM(Read-Only Memory), a RAM (Random Access Memory), a flash memory, amemory card, a storage medium, and/or other storage device. The RF unit1630, 1680 may include one or more antennas that transmit and/or receiveradio signals.

When an embodiment is implemented in software, the above-describedschemes may be embodied in modules (processes, or functions, etc.)performing the above-described functions. The modules may be stored inthe memory 1620, 1670 and may be executed by the processor 1610, 1660.The memory 1620, 1670 may be positioned in or outside the processor1610, 1660 and may be connected with the processor 1610, 1660 viavarious well-known means.

What is claimed is:
 1. A method for transmitting a clear to send (CTS)frame in a wireless local area network (WLAN), the method comprising:receiving, by a first station (STA), a multi-user request to send (MURTS) frame from an access point (AP), wherein the MU RTS frame istransmitted for soliciting simultaneous uplink transmission by the firstSTA and a second STA, wherein the MU RTS frame includes: firstassociation identifier (AID) information for the first STA, second AIDinformation for the second STA, first resource allocation informationfor a first uplink channel allocated to the first STA, and secondresource allocation information for a second uplink channel allocated tothe second STA; and transmitting, by the first STA, a first CTS frame tothe AP, wherein a second CTS frame is transmitted simultaneously withthe first CTS frame by the second STA, wherein the first CTS frame andthe second CTS frame are transmitted in response to the MU RTS frame,wherein the first CTS frame is transmitted via the first uplink channelbased on the first resource allocation information, and wherein thesecond CTS frame is transmitted via the second uplink channel based onthe second resource allocation information.
 2. The method of claim 1,further comprising: receiving, by the first STA, a plurality of dataframes after transmitting the first CTS frame, wherein the plurality ofdata frames are received by the second STA after transmitting the secondCTS frame.
 3. The method of claim 2, wherein a time when thetransmission of the plurality of data frames ends is the same as a timewhen transmission of a maximum interval transmission data frame ends,wherein the maximum interval transmission data frame is a frame in whicheffective downlink data is transmitted by the AP during the longestinterval, and wherein residual data frames other than the maximuminterval transmission data frame among the plurality of data frames arenull-padded.
 4. The method of claim 3, wherein the null-padded frameincludes a repeated MPDU delimiter field in which a MAC protocol dataunit (MPDU) length field is configured as
 0. 5. The method of claim 2,wherein each of the plurality of data frames includes at least one longtraining field (LTF) used for channel prediction, wherein the number ofLTFs included in each of the plurality of data frames is determinedbased on the largest number of spatial streams, and wherein the spatialstream is used to transmit at least two data streams among the pluralityof data frames to a specific STA based on multiple input multiple output(MIMO).
 6. A first station (STA) for transmitting a clear to send (CTS)frame in a wireless local area network (WLAN), the first STA comprising:a transceiver that transmits and receives a radio signal; a processorcoupled to the transceiver, wherein the processor is configured to:receive a multi-user request to send (MU RTS) frame from an access point(AP), wherein the MU RTS frame is transmitted for solicitingsimultaneous uplink transmission by the first STA and a second STA,wherein the MU RTS frame includes: first association identifier (AID)information for the first STA, second AID information for the secondSTA, first resource allocation information for a first uplink channelallocated to the first STA, and second resource allocation informationfor a second uplink channel allocated to the second STA; and transmit afirst CTS frame to the AP, wherein a second CTS frame is transmittedsimultaneously with the first CTS frame by the second STA, wherein thefirst CTS frame and the second CTS frame are transmitted in response tothe MU RTS frame, wherein the first CTS frame is transmitted via thefirst uplink channel based on the first resource allocation information,and wherein the second CTS frame is transmitted via the second uplinkchannel based on the second resource allocation information.
 7. Thefirst STA of claim 6, wherein the processor is configured to a pluralityof data frames after transmitting the first CTS frame, wherein theplurality of data frames are received by the second STA aftertransmitting the second CTS frame.
 8. The first STA of claim 7, whereina time when the transmission of the plurality of data frames ends is thesame as a time when transmission of a maximum interval transmission dataframe ends, wherein the maximum interval transmission data frame is aframe in which effective downlink data is transmitted by the AP duringthe longest interval, and wherein residual data frames other than themaximum interval transmission data frame among the plurality of dataframes are null-padded.
 9. The first STA of claim 8, wherein thenull-padded frame includes a repeated MPDU delimiter field in which aMAC protocol data unit (MPDU) length field is configured as
 0. 10. Thefirst STA of claim 7, wherein each of the plurality of data framesincludes at least one long training field (LTF) used for channelprediction, wherein the number of LTFs included in each of the pluralityof data frames is determined based on the largest number of spatialstreams, and wherein the spatial stream is used to transmit at least twodata streams among the plurality of data frames to a specific STA basedon multiple input multiple output (MIMO).